A typical automobile air conditioner includes a compressor, a condenser, an expansion valve, and an evaporator. The compressor compresses a cool vapor-phase refrigerant (e.g., freon, R134a) to heat the same, resulting in a hot, high-pressure vapor-phase refrigerant This hot vapor-phase refrigerant runs through a condenser, typically a coil that dissipates heat. The condenser condenses the hot vapor-phase refrigerant into liquid refrigerant. The liquid refrigerant is throttled through the expansion valve, which evaporates the refrigerant to a cold, low-pressure saturated liquid-vapor-phase refrigerant. This cold saturated liquid-vapor-phase refrigerant runs through the evaporator, typically a coil that absorbs heat from the air fed to the passenger compartment.
An automobile air conditioner consumes much engine power, which negatively impacts the acceleration performance and fuel economy. Attempts have been made to improve the air conditioner's efficiency by capturing some of the energy released by the hot, high-pressure refrigerant during the expansion stage, and applying the recovered energy toward compressing the cool vapor-phase refrigerant.
When a high-pressure, liquid refrigerant is throttled through an expansion valve or an orifice, it is transformed into a cold low-pressure saturated liquid-vapor-phase refrigerant, which is known as a "refrigeration effect." The throttling process itself does not fundamentally change the enthalpy (energy) content of the liquid-phase refrigerant. The liquid-phase to saturated liquid-vapor phase transformation, however, creates a boiling effect that liberates much kinetic energy, lowering the temperature of the refrigerant. The refrigerant's pressure drop from the high side to the low side and its subsequent expansion during cavitation (liquid-phase to saturated liquid-vapor-phase) provides excellent opportunity to extract mechanical work. Further, extracting work from the refrigerant will enhance the refrigeration cycle performance, since the energy content of the refrigerant is reduced. It would be desirable to capture this kinetic energy as much as possible.
In this regard, Japanese Patent publication Nos. 11-063707, 4-340062, and 61-96370, for example, disclose substituting the expansion valve with an expansion machine to capture part of the kinetic energy liberated during the throttling process. The expansion machine is essentially a motor driven by the hot, high-pressure liquid-phase refrigerant as it evaporates to a cold, low-pressure saturated vapor-phase refrigerant. The motor in turn is connected to a supercharger or compressor that can partially compress all or some of the cool vapor-phase refrigerant exiting from the evaporator, upstream of the compressor. The compressed refrigerant is fed through the compressor or fed to the condenser. Ideally, this should reduce the energy required to compress the refrigerant, thus making the air conditioner more efficient.
The present inventor has discovered that work can be best captured when the refrigerant is undergoing transformation from a liquid phase (or saturated liquid-vapor phase) to a saturated liquid-vapor phase having a higher vapor content, which occurs in a "high cavitation" region. Keeping the refrigerant in a high cavitation region within the motor, however, is difficult. The present inventor has discovered a way of maintaining the location of the high-cavitation region as the refrigerant is passed through an energy recovery device.